1. Field of the Invention
This invention relates to energy storage devices.
2. Description of the Related Art
(Note: This application references a number of different references as indicated throughout the specification by one or more reference numbers in superscripts, e.g., x. A list of these different references ordered according to these reference numbers can be found below in the section entitled “References.” Each of these references is incorporated by reference herein.)
Electric double-layer capacitors (EDLCs), interchangeably referred to as supercapacitors or ultracapacitors, are capable of storing and discharging energy quickly due to a physical ion adsorption/desorption mechanism in the Helmholtz layer.1,2 Compared to EDLCs using non-aqueous electrolytes, EDLCs with nonflammable aqueous electrolytes are in principle safer, and provide higher power density due to their having a lower ionic resistance.1,3-5 However, the energy density of aqueous EDLCs is limited by the narrow electrochemical potential window of water compared to counterparts with organic electrolytes or ionic liquids.6-8 The grand challenge for aqueous EDLCs is to increase specific energy without compromising specific power and cycling stability. A number of hybrid and pseudocapacitive systems have been developed that utilize different charge storage mechanisms in addition to, and/or in place of, electric double-layer capacitance to increase energy density.9-15 
One such approach to enhance the energy density is to replace the inert electrolytes of conventional EDLCs with redox-active electrolytes that enable faradaic charge storage.16-23 High-energy redox electrochemical capacitors (ECs) must concurrently utilize a catholyte and an anolyte (dual-redox enhanced electrochemical capacitors; dual-redox ECs) to maximize faradaic energy storage.19,23,25,26,36 Compared with the construction of nanostructured solid-state redox-active electrode materials, liquid-state redox-active electrolytes are easier to prepare and be scaled up, and should be compatible with the carbon electrodes that are currently mass-produced for commercial EDLCs.
In order to design redox-active electrolyte systems, redox couples should exhibit fast and reversible electron transfer and not engage in irreversible side reactions and/or degradation over repeated charge/discharge cycles. Additionally, the cross-diffusion of soluble redox couples that causes low Coulombic efficiency and fast self-discharge must be eliminated. The use of ion-selective membrane separators to mitigate self-discharge has been reported,24 but such membranes are costly, and primarily for that reason are not practical for commercial applications. Electrostatic attraction has also been proposed as a mechanism to retain redox couples at the surface of oppositely charged electrodes, but was shown to have only a minor effect on the suppression of self-discharge rates.25,26 
Halogens (I− and Br−) are promising aqueous redox-active species as they are inexpensive, electrochemically reversible redox couples with high solubility.22,25,27-31 In comparison to iodide, bromide has a higher standard reduction potential (0.81 V vs SCE; EI3−/I−o=0.3 V) that further increases energy density. However, this advantage is offset by the corrosive and volatile nature of bromine generated at the positive electrode. Furthermore, soluble Br3− diffuses to the negative electrode, which causes low Coulombic efficiency and fast self-discharge, as well as possible irreversible oxidation or bromination of the anode or anolyte.32,33 In aqueous bromine flow batteries, asymmetric quaternary ammonium salts such as methyl ethyl pyrrolidinium bromide (MEPBr; 1-ethyl-1-methylpyrrolidinium bromide) or methyl ethyl morpholinium bromide (MEMBr; 4-ethyl-4-methylmorpholinium bromide) are commonly used to complex Br2/Br3− as an oily-liquid phase.34,35 This complexation reduces the reactivity and vapor pressure of bromine while maintaining a mobile, liquid state for flow-system compatibility. However, this approach does not address the cross-diffusion and poor Coulombic efficiency.32 Standard bromine flow batteries avoid these limitations by storing the charged liquid complex in a separate tank away from the cell stack, but this practice is not feasible for non-flow systems, such as in redox-enhanced electrochemical capacitors (redox ECs). Alternatively, ion-exchange membrane separators can be used, but these materials are expensive and require addressing significant sealing challenges to be effective in a practical device. In short, a fundamental need is to store Br3− in non-flow energy storage systems in a manner that (1) reduces the unwanted chemical reactivity and vapor pressure of bromine but at the same time (2) suppresses cross-diffusion and self-discharge by (3) a simple and affordable mechanism. Embodiments of the present invention satisfy this need.